It is known in the art of microscopy to observe phase variations in an image of a sample in order to enhance detection of features that would otherwise be difficult to see. For example, methods of differential interference contrast (DIC) microscopy are described by M. Bass, E. W. Van Stryland, D. R. Williams, W. L. Wolfe in HandBook of Optics II (Second Edition, McGraw Hill, 1995), pages 17.28-17.36, which are incorporated herein by reference. DIC microscopy provides a shadow cast image that effectively displays the gradient of optical paths. Those regions of the sample where the optical paths increase along a certain reference direction appear brighter, while those where the path differences decrease appears in reverse contrast. Image contrast is greater the steeper the gradient of path differences. DIC methods are useful for highlighting features such as very thin filaments and sharp interfaces, and show differences in local refractive index, as well as changes in surface elevation.
Traveling lens acousto-optic devices are also known in the art. A device of this sort is described, for example, by Eveleth in U.S. Pat. No. 3,851,951, whose disclosure is incorporated herein by reference. An acoustic transducer is coupled to one end of an acousto-optic Bragg cell. The acoustic transducer generates frequency-modulated acoustic pulses in the Bragg cell, which travel from one end of the cell to the other. The resulting spatial frequency variation of the traveling acoustic pulse causes a laser beam that passes through the pulse area to be focused onto an image plane. As the acoustic pulse travels from one end of the Bragg cell to the other, it acts as a traveling lens, causing the focused laser spot to be scanned across the image plane.
It is known in the art that when a patterned object, such as a wafer, mask and the like, is illuminated with coherent radiation spots, such as but not limited to light spots, whereas each light spot is relatively large in relation to the repetitive pattern period, the coherent light is diffracted from the repetitive pattern and generates an interference pattern that is characterized by constructive interference lobes along well defined directions. The position and extent of the interference lobes depend on the period of the repetitive pattern, as well as the wavelength of the incident radiation and characteristics of the optical system. These interference lobes (also termed bright fringes) may prevent the detection of defects, either by masking scattered light from defects, by saturating the detector or by reducing the inspection system sensitivity to light scattered from defects.